1. Field
The following description relates to a PWM signal generating circuit for a DC-DC converter using a dimming signal and an LED driver circuit using the same, and, more specifically, a PWM signal generating circuit for a DC-DC converter which is capable of reducing a voltage ripple by generating a dimming signal which synchronizes with an internal clock, and an LED driver circuit using the same.
2. Description of the Related Art
An LCD (Liquid Crystal Display) type of display apparatus is thin and light compared to other types of display apparatuses. An LCD has a low driving voltage and consumes little electric power, and thus had become popular and widely used. However, since an LCD is a non-light-emitting element, and thus cannot emit light by itself, a separate backlight is used for providing light to an LCD panel.
Conventionally, a Cold Cathode Fluorescent Lamp (CCFL) and a Light Emitting Diode (LED) are two elements that are used as the aforementioned backlight source of an LCD. However, since the CCFL uses mercury, it may cause environmental pollution, as well as having a low speed response. In addition, the CCFL has other disadvantages such as poor color reproduction and not being appropriate to realize an LCD panel which has a high density, small size, and thin shape.
On the other hand, the LED does not use a material harmful to the environment, and is capable of performing with faster response time as compared to the CCFL. In addition, the LED has the advantages of good color reproduction and an ability to adjust brightness and color temperature arbitrarily by adjusting the amount of light of red, green, and blue LEDs. In addition, the LED is suitable for realizing an LCD panel of a high density, small size, and thin shape. Therefore, the LED has become more widely used as a light source for a backlight of an LCD panel in recent years.
As such, in conventional applications, if an LCD backlight using an LED employs a plurality of LEDs connected in a serial fashion, a driving circuit to provide constant current to the LEDs is required, and a dimming circuit to arbitrarily adjust brightness and color temperature arbitrarily, or perform temperature compensation, is required.
In more detail, different dimming methods used in an LCD backlight using an LED include an analog dimming method and digital dimming methods. The analog dimming method includes adjusting the brightness of the LED by adjusting an amount of electric current supplied to each of the LEDs. A pulse width modulation (PWM) dimming method, which is one of the digital dimming methods, includes adjusting the brightness of the LED by adjusting a ratio of on-off time of each LED. For example, if a PWM signal having an on-off time ratio of 4:1 is provided to each LED, the brightness of the LED reaches 80% of maximum brightness.
In order to adjust the brightness of the LED in the above-described digital dimming method, a clock signal of a DC-DC converter for adjusting power of the LED and a dimming signal for adjusting an amount of electric current in the LED are separately provided. In general, the frequency of the clock signal of the DC-DC converter is relatively high, and the frequency of the dimming signal is relatively low, and the clock signal of the DC-DC converter and the dimming signal are not synchronized with each other. Further, an on-period of the clock signal and the dimming signal are typically very short relative to the overall period of the respective signals.
Since, as discussed above, the clock signal and dimming signal of the DC-DC converter are not synchronized with each other, there is a problem of the actual time difference (tdr) between the clock signal and digital dimming signal changing at every cycle. This matter will be explained hereinafter in more detail with reference to FIG. 6.
FIG. 6 illustrates waveforms occurring in operations of generating a PWM signal based on a conventional dimming signal. Referring to FIG. 6, ‘B_CLK’ is a boosting clock signal, and ‘FPWM’ is a dimming signal generated inside an LED driver circuit according to an external control signal. In addition, ‘PWM_OUT’ is a PWM signal which is provided to a gate of a power transistor of a DC-DC converter.
With reference to FIG. 6, since the clock signal (B_CLK) and the dimming signal (FPWM) are not synchronized with each other, it can been seen that a delay value (tdri, the time from a rising edge of the dimming signal to a declining edge of the next clock signal, two examples of which being identified in FIG. 6 as tdr1 and tdr2) changes at every FPWM cycle. As such, there is a problem that in the case that such a PWM signal is not output periodically as aforementioned, the ripple of the output voltage of the DC-DC converter is increased.